Emulsion means a liquid-liquid dispersion system having a liquid phase in which at least one liquid phase immiscible with the former liquid phase is dispersed, and generally has a size distribution ranging from several micrometers to several tens micrometers.
Macroemulsion having such a particle size is in a thermodynamically unstable state, and tends to undergo separation by way of various paths such as flocculation, sedimentation, creaming, Ostwald ripening and coalescence.
When the emulsion particle size of a dispersed phase is lowered to a nano-scale, it is possible to improve the emulsion stability significantly in terms of kinetics through the Brownian movement among the particles, and to produce a low-viscosity emulsion having a high inner phase content. As a result, it is possible to provide a cosmetic agent having various feelings of use and to deliver active ingredients effectively to skin by virtue of such a small particle size.
In general, although there is a slight difference among various references, a liquid-liquid dispersion system in which particles of a dispersed phase has an average particle diameter of 20-500 nm is referred to as nanoemulsion (Flockhart, I. R. etc., Nanoemulsions derived from lanolin show promising drug delivery properties, J. Pharm. Pharmacol., 50 (Supplement) 1998, 141).
Many attempts have been made to produce a nanoemulsion having fine particles and low viscosity. Particularly, a phase inversion temperature (PIT) emulsification method or a high-pressure emulsification method has been used.
More particularly, a PIT emulsification method is one using the principle of hydrophilicity reduction in a ternary composition of water, oil and a non-ionic surfactant through a decrease in hydrogen bonding between ethylene oxide as a hydrophilic group of the non-ionic surfactant and water according to an increase in temperature. Herein, the ternary composition forms an O/W type emulsion below a specific temperature and forms a W/O type emulsion above a specific temperature. Such a specific temperature is called a PIT.
In addition, a high-pressure emulsification method is one including passing particles through a high-pressure emulsifier to which high shear is applied at 1000-1500 atm to provide nanoparticles. As a high-pressure emulsifier, a microfluidizer (U.S. Pat. No. 4,533,254) capable of application of a pressure of 1000 atm or higher is frequently used.
As described above, various attempts have been made to provide fine and uniform particles. However, they are limited technically in terms of viscosity, nano-sized particle formation, maximization in visual effects of white-colored cosmetic agents or cost, and thus cosmetic producers have many difficulties in making unlimited prescriptions. In fact, the methods according to the related art cannot find use for commercial production.
Under these circumstances, many studies have been conducted to overcome the above-mentioned technical limitation and to provide a nano-sized low-viscosity emulsion.